Neptunia oleracea (Water Mimosa) is a nitrogen-fixing aquatic perennial forming dense, floating mats of white spongy stems that spread rapidly over the water surface. The stems are clothed with finely pinnate, mimosa-like leaves that close up when slightly touched. The leaves have small oblong leaflets, 8-20 pairs per pinna, arranged in opposite pairs. Ball-shaped yellow inflorescences, packed with 30-50 tiny flowers bloom in summer (year-round in tropical areas). They give way to flat pods, 1-2 in. long (2-5 cm), containing 4-8 oval and brown seeds. On land, Water Mimosa has smaller leaves and flowers and no spongy floating tissue. Native to South America, Water Mimosa has naturalized in over 50 countries in South and Central America, Africa, and Asia, where it grows in wetlands, slow moving or still bodies of fresh water, and slow running tropical rivers. Water Mimosa is an economically important species in Thailand and Vietnam, where it is farmed in ponds and young shoots are harvested as a vegetable.

• Grows up to 3-6 in. tall (7-15 cm) and 1-5 ft. wide (30-150 cm).
• Performs best in full sun to part shade. It can be anchored in pots of soil or pea gravel at the ponds' edge, with some water over the crowns and with stems extending out into the water, but it is usually floated on the water.
• No serious pest or disease issues.
• Propagated by seeds and stem fragments that are dispersed by water.
• Monitor the growth and trim periodically as necessary.
• Overwinter indoors in an aquarium when outside fall temperatures drop to 50°F (10°C).
• The native habitat of Neptunia oleracea is unknown, but some experts believe it is in the area of Mexico to northern South America.
• Neptunia oleracea is considered to be an invasive aquatic weed in tropical zones as it produces dense, floating mats that can replace native wetland plants, block water flow in creeks and drains, reduce water quality, and kill native plants and fish. Water Mimosa is a restricted invasive plant in Australia under the Biosecurity Act 2014. In the United States, Neptunia is currently considered a noxious weed and not yet invasive.